I have the 8" that came out of my car sitting in the shed, I will see if I can get a measurement of drum to drum.
For what its worth I would keep the 8" for anything small block just put good parts in it and good axles.
No specific need to go to an 8.8 or a 9" in my opinion, as the ring gear won't be an issue in that type of racing.
Axles and retention of axles, and cooling would be important, and a good yoke with a decent size u-joint.
Some will argue that once you spend all the money on good parts for an 8" you could have bought a 9" or an 8.8 (plentiful and cheap) and that is a possibility minus of course the mods to shorten a diff or special wheels to make it work.
For sure so price it all out first.
So, If money is no object I would go 9" for sure even just for pure availability and selection of parts for it, if not put some good parts in the 8" and go race..